The present invention relates generally to a unique construction for a thread forming screw for cold forming internal threads in a workpiece comprised of a low ductility material, such as magnesium, or the like.
The general construction of screws and other threaded fasteners for cold forming complementary internal threads in a workpiece is well known in the relevant art, as is evident, for example, from the U.S. Pat. No. 3,942,406, to Enger, and the U.S. Pat. No. 3,935,785 to Lathom. These patents are assigned to the assignee of the present invention, and the disclosures thereof are incorporated herein by reference.
The prior art screws utilize relieved areas or interruptions in the thread turns or convolutions for facilitating cold forming of internal, female, complementary threads in the workpiece, thereby reducing the torque needed to drive the screw into the unthreaded workpiece bore, and providing open areas into which the cold formed metal can flow. Commonly, the relieved areas are formed by utilizing a polygonal blank having a finite number of shank sides onto which the external threads are rolled, or otherwise formed. As the threads are rolled across substantially non-circular flats of the shank, the resulting threads are interrupted, and the relieved areas are formed along the resulting thread helix.
The relieved areas along the thread helix allow the screw to swage or cold form workpiece material in the formation of internal threads in the workpiece by facilitating flow of workpiece material about the external threads of the screw as the screw is driven into the workpiece. Accordingly, no material is cut or removed from the workpiece by the cold forming of the threads; instead the material is swaged, reworked and reshaped into a proper configuration to form the complementary internal threads.
According to prior art conventions, these self-tapping, thread forming screws are often provided with threads defining a sixty degree included angle or angular configuration. This particular angular thread configuration works well with most workpiece materials. It should be noted, however, that this sixty degree thread configuration does not function well with all material. Specifically, it has been determined that sixty degree threads do not function well in forming internal threads in a workpiece comprised of a low ductility material, such as magnesium and the like. This fact and the inability to attain satisfactory fastening with state-of-the-art fastener systems has deterred designers and engineers from using these low ductility materials, despite substantial weight-to-strength advantages over more commonly used materials.
FIG. 5 somewhat diagrammatically depicts an external male thread of the general type and kind used to cold form a mating female thread in a workpiece of the prior art conventional sixty degree thread, as well as what is believed to be the forces acting on the male and female threads during the formation process. The resultant force acting upon the external and internal threads has a radial component directed along a radius of a pilot aperture formed in the workpiece, and an axial component directed along an axis of elongation of the aperture. Please note, that the above is based upon the assumption that oppositely directed forces of similar or same magnitude act upon the threads of the screw, and the workpiece as the screw is driven into the pilot aperture. It is to be noted that the axial component has a magnitude substantially greater than a corresponding magnitude of the radial component.
When it is attempted to form these conventional, sixty degree threads in a workpiece comprised of a low ductility material, such as magnesium, the above-discussed forces adversely affect the structural integrity of the internal threads thus formed, thereby resulting in galling, slivering, layer eruption and chipping of the workpiece material. The workpiece, or, more specifically, portions thereof adjacent the pilot aperture crumble and tear. Additionally, slivers can form proximate crests of the internal threads which can break away, thereby further impairing the structural integrity of the internal threads and interfering with driving of the male thread component.
Furthermore, stress fractures can form proximate the bases of the internal threads. The relatively increased magnitude of the axial component of the resultant force acting upon the internal threads can increase the propensity of the threads to fracture and shear off of the workpiece. The internal threads, therefore, have significantly reduced load bearing potential and reusability. The threads may also strip easily. FIGS. 6 and 7 show a simulated representation of how sixty degree internal threads may appear when cold formed by a prior art thread forming screw profile in a low ductility material workpiece.
According to the conventional wisdom and teachings of the prior art, one would attempt to solve these problems by utilizing a thread forming screw with a sharper thread profile. The teaching of the prior art would expect a beneficial result by using a thread form defining an angular configuration of less than sixty degrees to reduce the amount of galling. It has been determined empirically and by experiment, however, that this prior art approach does not solve the aforementioned problems.
Utilizing screw threads of less than sixty degrees to cold form internal threads in a workpiece causes thread forming stresses to concentrate in smaller and smaller areas in the internal threads. Because the material of the workpiece has a low ductility, the shear strength thereof is often correspondingly reduced. The concentration of the thread forming stresses, produced by the decreased angular configuration of the screw threads, can exceed the shear strength of the workpiece material. Accordingly, galling, chipping, and other breaches of structural integrity, similar to those described above, can occur. Therefore, the internal thread problems cannot be solved by reducing the angle of the angular configuration of the screw threads.
It is believed, however, that low ductility materials can accept a compressive force more readily, and with less destructive effects, than a shearing force, viz. the compressive strength of the material is greater than the shear strength. Therefore, if one were able to exploit this phenomenon, one might be able to avoid and possibly solve the internal thread problems presented by the prior art screws.
During cold formation of internal threads in a workpiece, the thread forming screw of the present invention produces a resultant force comprised of a radial component having a substantially greater magnitude than an axial component. This force relation is produced by the present invention by utilizing threads having an angular configuration defining an angle substantially within the range of ninety to one hundred and twenty degrees. The present invention employs an included thread angle which is greater, not less than the conventional sixty degree thread angle. The shear resisting area of the internal threads formed by the invention is substantially equal to the corresponding area of the internal threads formed by the prior art sixty degree screws. Accordingly, the internal threads formed by the invention, while of lesser depth, have equal or greater structural integrity and provide reduced thread stripping propensities and increased load bearing capacities.